Currently, fossil fuels, such as coal and oil fuels, are the main energy sources world widely. Besides sulfur oxides (SOx), nitrogen oxides (NOx, particularly NO and NO2) are included in the flue gas generated by coal combustion. There is also a high proportion of NOx in the flue gas of oil fuels combustion, such as motor gasoline and motor diesel.
Presence of NOx leads to a variety of serious environmental problems, such as photochemical smog, acid rain, greenhouse effect and damage of ozonosphere, etc. Meanwhile, NOx has bio-respiration toxicity, and is harmful to environment and human health.
With the shortage of petroleum resources and increasing pressure on reducing carbon dioxide emissions, diesel vehicles attract more and more attention due to its good fuel economy and drivability. Comparing with gasoline vehicles with three-way catalysts, exhaust pollution characterized by NOx and PM becomes the bottleneck in the development of diesel vehicles. Diesel vehicles have became the main resource of NOx and PM emissions from motor vehicles in China, and the key problem and difficulty in treating vehicle exhaust. The exhaust from diesel vehicles is characterized with low temperature, high oxygen, a large amount of particles and a little sulfur, etc, which makes purification and removal difficult.
At present, the main techniques for purifying NOx in diesel vehicles exhaust include direct decomposition of NO, NOx storage-reduction (NSR), hydrogen carbons selective catalytic reduction of NOx (HC-SCR), and NH3 selective catalytic reduction of NOx (NH3-SCR).
NO direct catalytic decomposition technique was emerged in last century, had once been thought as the most ideal method to remove lean-burn NOx. Theoretically, NO is thermally unstable, but its activation energy of decomposition reaction can reach 364 kJ/mol. To promote the reaction, appropriate catalysts should be chosen to reduce the activation energy barrier. Existing researches have shown that many catalysts, such as noble metals, metal oxides, zeolites, etc, can promote the decomposition of NO. But the presence of oxygen may inhibit NO decomposition reaction, and oxygen desorption is the limit step of the whole process. Accordingly, it is very difficult to apply NO direct catalytic decomposition technique in practice for treating lean-burn exhaust.
NSR technique is based on excellent capability of three-way catalysts for removing HC and NOx simultaneously, and cooperating with the NOx adsorbent to trap NOx in the lean phase; achieving the aim of removing HC and NOx simultaneously by regulating the engine to the rich phase periodically and reducing NOx by HC in the exhaust. However, for the implementation of this technique, engine condition should be controlled precisely, and lean and rich conditions should be operated alternatively, to make the catalyst exhibit the best NOx removal efficiency, which increases the difficulty for controlling the engine; at the same time, operating at rich condition increases fuel consumption, and reduces the fuel economy of diesel engine; additionally, the sensitivity of NSR catalysts to sulfur limits its application.
HC-SCR selectively catalyzes the reduction of NOx with hydrocarbons. Generally, alkanes and alkenes are used to reduce NOx on catalysts with high selectivity. The catalysts may be divided into 3 categories: (1) metal ion-exchanged molecular sieve catalysts, including ZSM series, ferrierite, mordenite, silicon aluminum phosphate molecular sieve (SAPO), Y-type zeolites, L-type zeolites, etc; (2) non-noble metal oxide catalysts, including loaded metal oxide catalysts with a carrier, such as Al2O3, SiO2, TiO2, ZrO2, and so on, bimetallic catalysts comprising of Al2O3, SiO2, TiO2, ZrO2, Cr2O3, Fe2O3, CO3O4, CuO, V2O5, Bi2O3, MgO, and so on, rare earth perovskite mixed oxides, such as LaAlO3; (3) noble metal catalysts, such as Pt, Pd, Rh, Au, and so on, in atom type, or exchanged on zeolites, or loaded on Al2O3, SiO2, TiO2, ZrO2. Catalysts used in the HC-SCR method need carriers to disperse catalytic active components and increase the specific surface area, which make the catalysts with carrier have larger specific surface area with the same active components. Thus, the catalysts need large space when the method is applied in limited fields such as diesel vehicles.
NH3-SCR is thought to be the most promising technology that can be applied widely to purify exhaust gases of diesel vehicles. At present, this technology has been taken into practice, which is the most world-widely used flue gas deNOx technology. Generally, NH3-SCR catalysts used in industry are V2O5-WO3 (MoO3)/TiO2 catalysts containing toxic vanadium (V). The catalysts not only need carriers, but also V may be detached and enter into environment, when the active components are working. V5+ pollutes the environment, and further is harmful to humans, for its high bio-toxicity. Accordingly, the usage of V5+ is limited in Europe and America. At the same time, this catalyst system has disadvantages such as narrow operation temperature range, being prone to catalyze SO2 in flue gas to SO3, and so on.
Therefore, it is very environmentally important for developing novel nontoxic vanadium-free catalyst system with high NH3-SCR activity, broad operation temperature range, suitable for high space velocity conditions, which catalyze and remove nitrogen oxides from mobile sources represented by diesel engine exhaust gases and stationary sources represented by flue gas of coal-fired power plants.